1. Field of the Invention
This document relates to a liquid crystal display device, which can improve display grade, and a driving method thereof.
2. Related Art
A liquid crystal display device controls the light transmittance of a liquid crystal layer by an electric field applied to the liquid crystal layer in accordance with video signals to display a picture. As the liquid crystal display devices are thin and flat panel display devices having low power consumption, the liquid crystal display devices are used as displays for portable computers such as laptop computers, office automation devices, audio/video devices, and the like. Especially, an active matrix type liquid crystal display device where a switching device is formed for each liquid crystal cell is advantageous in realizing motion pictures because the switching device can be actively controlled.
The switching device used in the active matrix type liquid crystal display device is mainly a thin film transistor (hereinafter, referred to as “TFT”), as in FIG. 1.
Referring to FIG. 1, an active matrix type liquid crystal display device converts digital video data into analog data voltages on the basis of a gamma reference voltage to supply to data lines DL, and at the same time, supplies scan pulses to gate lines GL to charge liquid crystal cells Clc therewith. The TFT includes a gate electrode connected to the gate line GL, a source electrode connected to the data line DL and a drain electrode connected to a pixel electrode of the liquid crystal cell Clc and one electrode of a storage capacitor Cst1. Common voltages Vcom are supplied to a common electrode of the liquid crystal cell Clc. When the TFT is turned on, the storage capacitor Cst is charged with the data voltages applied from the data line DL, to fixedly maintain the voltage of the liquid crystal cell Clc. If the scan pulses are applied to the gate line GL, the TFT is turned on to form a channel between the source electrode and the drain electrode, thereby supplying the voltage of the data line DL to the pixel electrode of the liquid crystal cell Clc. At this moment, the liquid crystal molecules of the liquid crystal cell Clc are changed in arrangement by the electric field between the pixel electrode and the common electrode, thereby modulating the incident light.
However, when a DC voltage is applied for a long time to the liquid crystal layer of the liquid crystal display device, ions having negative charges are moved in the same motion vector direction and ions having positive charges are moved in the opposite motion vector direction in accordance with the polarity of an electric field applied to the liquid crystal, and polarized, and the accumulated amount of the ions having negative charges and the accumulated amount of the ions having positive charges are increased with time. As the accumulated amount of the ions increases, the orientation layer is deteriorated, and as a result, the orientation characteristic of the liquid crystal is deteriorated. Due to this, when a DC voltage is applied for a long time to the liquid crystal display device, a blur appears on a displayed image, and the blur becomes larger with time. To overcome this blur, a method of developing a liquid crystal material having a low dielectric constant or a method of improving an orientation material or orientation method has been attempted. However, this method requires a lot of time and cost to develop materials, and the lowering of the dielectric constant of liquid crystal may cause another problem of deterioration of the driving characteristics of the liquid crystal. According to experimentally obtained results, the more the impurities to be ionized in the liquid crystal layer, and the higher the acceleration factors, the faster the point of time of blur appearance. The acceleration factors include temperature, time, DC driving of liquid crystal, etc. Accordingly, the higher the temperature or the longer the period of time for applying a DC voltage of the same polarity to the liquid crystal layer, the faster a blur appears and the more severe the degree of the blur. Moreover, since a blur is different in shape and degree even in panels of the same model manufactured by the same manufacturing line, this cannot be solved only by developing a new material or by improving the process.